Surface acoustic wave (SAW) resonator

ABSTRACT

A surface acoustic wave (SAW) resonator device is disclosed. The SAW resonator has a piezoelectric layer disposed over a semiconductor substrate. The piezoelectric layer has a first surface and a second surface. The polarization axis (C-axis) of the piezoelectric layer is oriented at an angle relative to the second surface of the piezoelectric layer in a range of approximately 0° to approximately 45°. A layer is disposed between the first surface of the semiconductor substrate and the second surface of the piezoelectric layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part under 37 C.F.R. § 1.53(b) of,and claims priority under 35 U.S.C. § 120 from, U.S. patent applicationSer. No. 14/835,679 filed on Aug. 25, 2015, naming Stephen Roy Gilbert,et al. as inventors; Ser. No. 14/866,394 filed on Sep. 25, 2015 namingStephen Roy Gilbert, et al. as inventors; Ser. No. 15/009,801, filed onJan. 28, 2016, naming Stephen Roy Gilbert, et al. as inventors; and U.S.patent application Ser. No. 15/245,392, filed on Aug. 24, 2016, namingStephen Roy Gilbert, et al. as inventors. The entire disclosures of U.S.patent application Ser. Nos. 14/835,679, 14/866,394, 15/009,801, andSer. No. 15/245,392 are specifically incorporated herein by reference.

BACKGROUND

Electrical resonators are widely incorporated in modern electronicdevices. For example, in wireless communications devices, radiofrequency (RF) and microwave frequency resonators are used in filters,such as filters having electrically connected series and shuntresonators forming ladder and lattice structures. The filters may beincluded in a duplexer (diplexer, triplexer, quadplexer, quintplexer,etc.) for example, connected between an antenna and a transceiver forfiltering received and transmitted signals.

Various types of filters use mechanical resonators, such as surfaceacoustic wave (SAW) resonators. The resonators convert electricalsignals to mechanical signals or vibrations, and/or mechanical signalsor vibrations to electrical signals.

SAW resonators can be provided over high-resistivity, monocrystallinesilicon substrates, so that radio-frequency (RF) losses due to currentsin the substrate generated by electric fields from the electrodes aremitigated. However, in spite of the use of a high-resistivity, undopedmonocrystalline silicon substrate, an inversion channel can be formeddue to charges in other layers in the SAW structure. Carriers in thesubstrate, in turn, can be injected into the inversion layer. Spuriouscurrents can result from these charges due to the electric fieldsgenerated by the electrodes. Thus, in known SAW structures, RF lossesdue to spurious currents can remain.

What is needed, therefore, is a SAW resonator device that overcomes atleast the shortcomings of known SAW resonators described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The example embodiments are best understood from the following detaileddescription when read with the accompanying drawing figures. It isemphasized that the various features are not necessarily drawn to scale.In fact, the dimensions may be arbitrarily increased or decreased forclarity of discussion. Wherever applicable and practical, like referencenumerals refer to like elements.

FIG. 1 is a top view of a SAW resonator device in accordance withrepresentative embodiments.

FIG. 2A is the cross-sectional view of a SAW resonator device inaccordance with a representative embodiment.

FIG. 2B is a conceptual view in cross-section of a portion of the SAWresonator device of FIG. 2A, showing the direction of the polarizationaxis of the piezoelectric material, and the electric field lines inaccordance with a representative embodiment.

FIG. 3 is a conceptual view of a cross-sectional view of a portion ofthe SAW resonator device in accordance with a representative embodiment.

FIG. 4 is a cross-sectional view of a portion of the SAW resonatordevice in accordance with a representative embodiment.

FIG. 5 is a simplified schematic block diagram of a filter comprisingSAW resonator devices, according to a representative embodiment.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, representative embodiments disclosing specific detailsare set forth in order to provide a thorough understanding of thepresent teachings. However, it will be apparent to one having ordinaryskill in the art having had the benefit of the present disclosure thatother embodiments according to the present teachings that depart fromthe specific details disclosed herein remain within the scope of theappended claims. Moreover, descriptions of well-known apparatuses andmethods may be omitted so as to not obscure the description of therepresentative embodiments. Such methods and apparatuses are clearlywithin the scope of the present teachings.

It is to be understood that the terminology used herein is for purposesof describing particular embodiments only, and is not intended to belimiting. Any defined terms are in addition to the technical andscientific meanings of the defined terms as commonly understood andaccepted in the technical field of the present teachings.

As used in the specification and appended claims, the terms ‘a’, ‘an’and ‘the’ include both singular and plural referents, unless the contextclearly dictates otherwise. Thus, for example, ‘a device’ includes onedevice and plural devices.

As used in the specification and appended claims, and in addition totheir ordinary meanings, the terms ‘substantial’ or ‘substantially’ meanto with acceptable limits or degree. For example, ‘substantiallycancelled’ means that one skilled in the art would consider thecancellation to be acceptable.

As used in the specification and the appended claims and in addition toits ordinary meaning, the term ‘approximately’ means to within anacceptable limit or amount to one having ordinary skill in the art. Forexample, ‘approximately the same’ means that one of ordinary skill inthe art would consider the items being compared to be the same.

Relative terms, such as “above,” “below,” “top,” “bottom,” “upper” and“lower” may be used to describe the various elements' relationships toone another, as illustrated in the accompanying drawings. These relativeterms are intended to encompass different orientations of the deviceand/or elements in addition to the orientation depicted in the drawings.For example, if the device were inverted with respect to the view in thedrawings, an element described as “above” another element, for example,would now be “below” that element. Similarly, if the device were rotatedby 90° with respect to the view in the drawings, an element described“above” or “below” another element would now be “adjacent” to the otherelement; where “adjacent” means either abutting the other element, orhaving one or more layers, materials, structures, etc., between theelements.

The described embodiments relate generally to surface acoustic wave(SAW) resonators. Generally, the SAW resonators comprise interdigitatedelectrodes disposed over a piezoelectric layer, which in turn isdisposed over a substrate. The piezoelectric layer has a polarizationaxis (C-axis) that is not oriented perpendicular to the substrate, butrather substantially parallel to the substrate (0° degrees relative to aplane of the substrate, as described more fully below), or at an anglethat is less than 45° degrees relative to the plane of the substrate.

In accordance with a representative embodiment, a surface acoustic wave(SAW) resonator device comprises: a semiconductor substrate having afirst surface and a second surface; a piezoelectric layer disposed overthe substrate, the piezoelectric layer having a first surface and asecond surface, wherein a polarization axis (C-axis) of thepiezoelectric layer is oriented at an angle in the range ofapproximately 0° to approximately 45° relative to the second surface ofthe piezoelectric layer; a plurality of electrodes disposed over thefirst surface of the piezoelectric layer, the plurality of electrodesconfigured to generate longitudinal acoustic waves in the piezoelectriclayer; and a layer disposed between the first surface of the substrateand the second surface of the piezoelectric layer.

In accordance with another representative embodiment, a surface acousticwave (SAW) resonator device, comprises: a semiconductor substrate havinga first surface and a second surface, the semiconductor substratecomprising a bulk region and a surface region wherein the surface regionhas a high trap density and a reduced carrier mobility compared to thebulk region; a piezoelectric layer disposed over the substrate, thepiezoelectric layer having a first surface and a second surface, whereina polarization axis (C-axis) of the piezoelectric layer is oriented atan angle relative to the first and second surfaces in the range ofapproximately 0° to approximately 45°; and a plurality of electrodesdisposed over the first surface of the piezoelectric layer, theplurality of electrodes configured to generate surface acoustic waves inthe piezoelectric layer.

In certain representative embodiments, the piezoelectric layer comprisesa piezoelectric material doped to a minimum atomic percentage with atleast one rare earth element. When percentages of doping elements in apiezoelectric layer or a buffer layer are discussed herein, it is inreference to the total atoms of the piezoelectric layer. Notably, whenthe percentages of doping elements (e.g., Sc) in a doped AlN layer arediscussed herein, it is in reference to the total atoms (includingnitrogen) of the AlN piezoelectric layer. So, for example, and asdescribed for example in U.S. Patent Application Publication No.20140132117, if the Sc in the piezoelectric layer of a representativeembodiment has an atomic percentage of approximately 5.0%, and the Alhas an atomic percentage of approximately 95.0%, then the atomicconsistency of the piezoelectric layer may then be represented asAl_(0.95)Sc_(0.05) N.

In certain embodiments described in more detail below, the piezoelectriclayer comprises aluminum nitride (AlN) that is doped with scandium (Sc).The atomic percentage of scandium in an aluminum nitride layer isapproximately greater than 9.0% to approximately 44.0%; and in otherrepresentative embodiments, the percentage of scandium in an aluminumnitride layer is approximately greater than 5.0% to approximately 44.0%.In such embodiments, and as described more fully below, in embodimentswith two buffer layers, each buffer layer also comprises AlN, and one orboth of the first and second buffer layers is doped with scandium to anatomic percentage less than that of the Sc-doped layer. Specifically, incertain representative embodiments, one or both of the first and secondbuffer layers is doped with scandium to an atomic percentage in therange of approximately 0.0% to less than 9.0%; and in otherrepresentative embodiments one or both of the first and second bufferlayers is doped with scandium to an atomic percentage in the range ofapproximately 0.0% to less than approximately 5.0%. As noted above, theatomic doping levels (atomic percentages) of scandium in the first andsecond AlN buffer layers are not necessarily the same. In otherembodiments described below, only one AlN buffer layer is provideddirectly on top of the piezoelectric layer. This buffer layer is dopedwith scandium to an atomic percentage less than the atomic doping levelof the Sc-doped AlN layer.

As described more fully below, in certain embodiments, the substrateused for the SAW resonator devices of the present teachings may comprisea high-resistivity monocrystalline semiconductor material. In otherembodiments, the substrate used for the SAW resonator device maycomprise a polycrystalline material, an amorphous material, or anelectrically insulating material.

In embodiments in which the substrate comprises a high-resistivitymonocrystalline semiconductor material (e.g., monocrystalline silicon),an oxide layer may exist between the piezoelectric material of the SAWresonator device, and the substrate. The periodic crystalline structureof the high-resistivity monocrystalline semiconductor material results,of course, in the establishment of valence and conduction bands in thesubstrate. Energy-band bending may occur at the interface of thehigh-resistivity monocrystalline semiconductor substrate and the oxidelayer, due to differences in the work-functions of the materials,charges deeply trapped in the oxide layer, and the surface polarizationof a piezoelectric layer disposed thereover.

As such, near the substrate-oxide interface, the conduction band edge isbrought close to the Fermi level, populating the surface region withcarriers (electrons or holes depending on the band bending) in aninversion channel at this interface. The free carriers that populate theinversion channel are then susceptible to the electric fields generatedby the electrodes of the SAW resonator device, and unwanted (spurious)RF currents are supported. These spurious currents can result innon-linear behavior of the SAW resonator device due to the formation ofpn junctions in the structure, and intermodulation distortion (IMD)products, which ultimately deteriorate the performance of the SAWresonator device, and devices (e.g., filters) in which the SAW resonatordevice is implemented.

As noted, the piezoelectric material used for the piezoelectric layermay be rare-earth doped aluminum nitride. When such piezoelectricmaterials are provided directly on the high-resistivity monocrystallinesemiconductor substrates, surface polarization can also cause thecreation of an inversion channel with free carriers, and the deleteriousspurious RF currents can result.

According to the present teachings, when a high-resistivitymonocrystalline semiconductor substrate is used, a region (referred toas the surface region) of material is disposed between the bulkhigh-resistivity monocrystalline semiconductor substrate (sometimesreferred to as the bulk region), and an oxide layer, or other suitabledielectrics, including, but not necessarily limited to silicon nitride(SiN), or silicon carbide (SiC). The surface region has high trapdensity, and a reduced carrier mobility, compared to thehigh-resistivity monocrystalline semiconductor substrate (sometimesreferred to as a surface region). As can be appreciated, the high trapdensity surface region presents a high probability of trapping (orannihilating) free charge carriers, which substantially reduces thecreation of spurious RF currents due to electric fields from theelectrodes of the SAW resonator device. Beneficially, therefore,undesired IMD products are reduced, and linearity of the SAW resonatordevices is improved compared to known devices.

In other embodiments, described below, the creation of an inversionchannel is avoided by using an amorphous, or a polycrystalline material(e.g., amorphous Si, or polycrystalline Si) as the substrate for the SAWresonator device. As described more fully below, the amorphous orpolycrystalline layer may be deposited over the bulk high-resistivitysubstrate, or formed by roughening the upper portion of the bulkhigh-resistivity substrate using a known technique (e.g., mechanicalgrinding), or by ion implanting the Si substrate using a knowntechnique, such as described in U.S. Pat. No. 7,728,485, the entiredisclosure of which is specifically incorporated herein by reference. Instill other embodiments, the substrate of the SAW resonator device maycomprise an electrically insulating material, including but notnecessarily limited to sapphire, glass, spinel, and alumina ceramic.

FIG. 1 is a top view of a SAW resonator device 100 according to arepresentative embodiment. Notably, the SAW resonator device 100 isintended to be merely illustrative of the type of device that canbenefit from the present teachings. Other types of SAW resonators,including, but not limited to dual mode SAW (DMS) resonators, andstructures therefore, are contemplated by the present teachings. The SAWresonator device 100 of the present teachings is contemplated for avariety of applications. By way of example, and as described inconnection with FIG. 5, a plurality of SAW resonator devices 100 can beconnected in a series/shunt arrangement to provide a ladder filter.

The SAW resonator device 100 comprises a piezoelectric layer 103disposed over a substrate (not shown in FIG. 1). In accordance withrepresentative embodiments, the piezoelectric layer 103 comprisesaluminum nitride (AlN) or rare-earth doped (AlN), such as Scandium-dopedAlN (ASN). As described more fully below, the C-axis of thepiezoelectric layer 103 is beneficially parallel to the plane of thepiezoelectric layer 103, or at a comparatively small angle therefrom.

The SAW resonator device 100 comprises an active region 101, whichcomprises a plurality of interdigitated electrodes 102 disposed over apiezoelectric layer 103, with acoustic reflectors 104 situated on eitherend of the active region 101. In the presently described representativeembodiment, electrical connections are made to the SAW resonator device100 using the busbar structures 105.

As is known, the pitch of the resonator electrodes determines theresonance conditions, and therefore the operating frequency of the SAWresonator device 100. Specifically, the interdigitated electrodes arearranged with a certain pitch between them, and a surface wave isexcited most strongly when its wavelength λ is the same as the pitch ofthe electrodes. The equation f₀=v/λ describes the relation between theresonance frequency (f₀), which is generally the operating frequency ofthe SAW resonator device 100, and the propagation velocity (v) of asurface wave. These SAW waves comprise Rayleigh or Leaky waves, as isknown to one of ordinary skill in the art, and form the basis offunction of the SAW resonator device 100.

Generally, there is a desired fundamental mode, which is typically aLeaky mode, for the SAW resonator device 100. By way of example, if thepiezoelectric layer 103 is a 42° rotated LT, the shear horizontal modehas a displacement in the plane of the interdigitated electrodes 102(the x-y plane of the coordinate system of FIG. 1). The displacement ofthis fundamental mode is substantially restricted to near the uppersurface (first surface 110 as depicted in FIG. 1) of the piezoelectriclayer 103. It is emphasized that the 42° rotated LT piezoelectric layer103, and the shear horizontal mode are merely illustrative of thepiezoelectric layer 103 and desired fundamental mode, and othermaterials and desired fundamental modes are contemplated.

FIG. 2A is a cross-sectional view of a SAW resonator device 200 inaccordance with a representative embodiment. Many aspects and details ofSAW resonator device 200 are common to those described above inconnection with FIG. 1. These common aspects and details may not berepeated to avoid obscuring the description of the presentrepresentative embodiment.

The SAW resonator device 200 comprises a substrate 208 disposed beneaththe piezoelectric layer 203, and a layer 209 disposed between thesubstrate 208 and the piezoelectric layer 203.

The piezoelectric layer 203 illustratively comprises aluminum nitride(AlN) or scandium doped AlN (ASN). Generally, in the representativeembodiments described below, the piezoelectric layer 203 is a wafer thatis previously fabricated, and is adhered to the layer 209 by atomicbonding as described more fully below.

While many of the representative embodiments relate to scandium-dopedMN, it is noted that other rare-earth dopants are contemplated fordoping the piezoelectric material of piezoelectric layer 203.Specifically, the other rare-earth elements include yttrium (Y),lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb) and lutetium (Lu). The various embodiments contemplateincorporation of any one or more rare-earth elements, although onlyspecific examples are discussed herein.

As described more fully below in connection with FIG. 2B, thepiezoelectric layer 203 has a polarization axis (C-axis) at an angle (Θ)in the range of approximately 0° to approximately 45° relative to thex-y plane of the coordinate system shown in FIG. 2A. In otherrepresentative embodiments, the piezoelectric layer 203 has apolarization axis (C-axis) at an angle (Θ) in the range of approximately0° to approximately 30° relative to the x-y plane of the coordinatesystem shown in FIG. 2A. As can be appreciated, the second surface 211of the piezoelectric layer 203 is disposed in the x-y plane, so theC-axis is at an angle (Θ) in the range of approximately 0° toapproximately 45°. Stated somewhat differently, the C-axis of thepiezoelectric layer 203 is an angle (Θ) subtended by the x and z axes ofthe coordinate system shown in FIG. 3 that is disposed in the x-y plane,so the C-axis is at the angle (Θ) in the range of approximately 0° toapproximately 45°, and in some embodiments in the range of approximately0° to approximately 30°.

In accordance with a representative embodiment, the substrate 208comprises a bulk region 208′, and a surface region 208″. As describedmore fully below, the bulk region 208′ comprises a high-resistivitymonocrystalline semiconductor material, and the surface region 208″ hasa high trap density and a reduced carrier mobility compared to the bulkregion 208′.

In accordance with a representative embodiment, the bulk region 208′comprises monocrystalline silicon, and has a thickness of approximately50.0 μm to approximately 800.0 μm. In certain representativeembodiments, in order to improve the performance of a filter comprisingSAW resonator device(s) 200, the substrate 208 may comprise acomparatively high-resistivity material. Illustratively, the substrate208 may comprise single-crystal silicon that is doped to a comparativelyhigh resistivity. Notably, other high-resistivity, monocrystallinematerials besides silicon are contemplated for use as the substrate 208of the SAW resonator device 200. By way of example, other single-crystalsemiconductor materials can be used for the substrate 208. Moreover,other single-crystal materials, such as single-crystal aluminum oxide(Al₂O₃) (sometimes referred to as “sapphire”) could be used as well.

The surface region 208″ comprises a material, which, compared to thebulk region 208′, has an increased bandgap, very high trap density (highprobability of trapping free charge carriers), and a reduced carriermobility.

The thickness of this (nonmonocrystalline) surface region 208″ should beat least as great as the depth of an inversion layer that would form ifthe surface region 208″ were made up of a monocrystalline material(e.g., silicon). Since the thickness of the inversion layer depends,among other things, on the semiconductor material and its doping, thethickness of the surface region 208″ is typically in range from a fewnanometers (nm) up to several hundreds of micrometers (μm). In certainrepresentative embodiments, the surface region 208″ comprises a layer ofamorphous semiconductor material having a thickness in the range ofapproximately 1 nm to approximately 700 μm.

As noted above, the bulk region 208′ may comprise single-crystalsilicon. In such an embodiment, the surface region may compriseamorphous silicon (aSi), or polycrystalline silicon. Notably, however,since a single grain of a polysilicon material is monocrystalline, thegrain size of the polycrystalline silicon here should be much smallercompared to a typical resonator device size and the thickness of thesurface region 208″. Again, in accordance with a representativeembodiment, an average grain size of more than 10 times smaller than athickness of the surface region is beneficial.

The surface region 208″ can be formed by one of a number of knownmethods. For example, an amorphous silicon layer can be deposited by aknown method over the bulk region 208′ to form the substrate 208.Similarly, a polysilicon layer may be deposited by a known method overthe bulk region 208′ to form the substrate 208. Notably, of course, thegrain size of the polysilicon structure that comprises the surfaceregion 208″ in such an embodiment should be much smaller than thestructure sizes/area of the resonator. Moreover, known ion implantationmethods may be used to dope or amorphize the monocrystalline structureof the substrate 208, thereby creating the surface region 208″ over thebulk region 208′.

As noted above, other semiconductor materials (not silicon) can be usedas the substrate 208. By similar methods the surface region 208″ can beprovided over the bulk region 208′ by depositing or forming adeteriorated lattice structure by similar techniques to those used forsilicon. As such, adding a layer of amorphous or polycrystallinematerial over the bulk region 208′, or deteriorating the latticestructure of a given substrate material provides a surface region 208″comprising an increased bandgap, very high trap density and an at least100 times reduced carrier mobility, compared to the bandgap, trapdensity and carrier mobility of the bulk region 208′.

As noted above, a benefit of certain representative embodiments is thatthe use of such a substrate avoids the generation of a surface channelat the surface of the semiconductor, which otherwise (for a conventionalsubstrate) is formed by an inversion layer. Such a surface channelresults in a lossy surface current between resonators and/orinterconnections of different potential. As a consequence, SAW resonatordevices of the present teachings will provide a comparatively lowerloss, which means that the SAW filters comprise an improved insertionloss, or the SAW resonators comprise an improved quality factor (Q).

The layer 209 is illustratively an oxide material, such as silicondioxide (SiO₂), or phosphosilicate glass (PSG), borosilicate glass(BSG), a thermally grown oxide, or other material amenable to polishingto a high degree of smoothness, as described more fully below. The layer209 is deposited by a known method, such as chemical vapor deposition(CVD) or plasma enhanced chemical vapor deposition (PECVD), or may bethermally grown. As described more fully below, the layer 209 ispolished to a thickness in the range of approximately 0.05 μm toapproximately 6.0 μm.

The piezoelectric layer 203 has a first surface 210, and a secondsurface 211, which opposes the first surface 210. Similarly, the layer209 has a first surface 212 and a second surface 213. As depicted inFIG. 2A, the first surface 212 of the layer 209 is atomically bonded tothe second surface 211 of the piezoelectric layer 203, as described morefully below.

As described more fully below in connection with FIG. 2B, thepiezoelectric layer 203 has a polarization axis (C-axis) at an angle (Θ)in the range of approximately 0° to approximately 45° relative to thex-y plane of the coordinate system shown in FIG. 2A. As can beappreciated, the second surface 211 of the piezoelectric layer 203 isdisposed in the x-y plane, so the C-axis is the angle (Θ) in the rangeof approximately 0° to approximately 45°. Stated somewhat differently,the C-axis of the piezoelectric layer 203 is an angle (Θ) subtended bythe x and z axes of the coordinate system shown in FIG. 2B that isdisposed in the x-y plane, so the C-axis is at the angle (Θ) in therange of approximately 0° to approximately 45°.

The substrate 208 has a first surface 214 and a second surface 215opposing the first surface 214. The first surface 214 has a plurality offeatures 216 there-across. Undesired spurious modes, which are bulkmodes, are launched in the piezoelectric layer 203, and propagate downto the first surface 214. As described more fully in above-incorporatedU.S. patent application Ser. No. 14/835,679, the plurality of features216 reflect undesired spurious modes at various angles and over variousdistances to destructively interfere with the undesired spurious wavesin the piezoelectric layer 203, and possibly enable a portion of thesewaves to be beneficially converted into desired SAW waves. Thereflections provided by the plurality of features 216 foster a reductionin the degree of spurious modes (i.e., standing waves), which arecreated by the reflection of acoustic waves at the interface of thesecond surface 211 of the piezoelectric layer 203 and the first surface212 of layer 209. Ultimately, the reflections provided by the pluralityof features 216 serve to improve the performance of devices (e.g.,filters) that comprise a plurality of SAW resonator devices 200.

As described in above-incorporated U.S. patent application Ser. No.14/835,679, the first surface 212 of layer 209 is polished, such as bychemical-mechanical polishing in order to obtain a “mirror” like finishwith a comparatively low RMS variation of height. This low RMS variationof height significantly improves the contact area between the firstsurface 212 of the layer 209 and the second surface 211 of thepiezoelectric layer 203 to improve the atomic bonding between the firstsurface 212 of layer 209 and the second surface 211 of the piezoelectriclayer 203. As is known, the bond strength realized by atomic bonding isdirectly proportional to the contact area between two surfaces. As such,improving the flatness/smoothness of the first surface 212 fosters anincrease in the contact area, thereby improving the bond of the layer209 to the piezoelectric layer 203. As used herein, the term “atomicallysmooth” means sufficiently smooth to provide sufficient contact area toprovide a sufficiently strong bond strength between the layer 209 andthe piezoelectric layer 203, at the interface of their first and secondsurfaces 212, 211, respectively.

As described more fully in above-incorporated U.S. patent applicationSer. No. 14/835,679, the shape, dimensions and spacing of the features216 depends on their method of fabrication. As will be appreciated byone of ordinary skill in the art, the method of fabricating the features216 will depend on when the surface region 208″ is formed. By way ofexample, if the features 216 are formed before the surface region 208″is formed, the substrate 208, at this point, may be monocrystallinematerial (e.g., moncrystalline Si). In this case, the features 216 maybe formed by selective etching, such as described in above-incorporatedU.S. patent application Ser. No. 14/835,679, or other methods describedtherein. However, if the surface region 208″ is formed before thefeatures 216 are formed (e.g., by ion implantation, deposition of anamorphous material or a polycrystalline material, or mechanicalgrinding), the features 216 cannot be formed by selective etching, andare thus formed by other known techniques, such as mechanical rougheningmethods. Still alternatively, the features 216 may be formed prior tothe formation of the surface region 208″. As such, the features 216 maybe formed by a selective etch, or other mechanical roughening methods,and the amorphous or polysilicon material may be deposited over thefeatures.

As noted in above-incorporated U.S. patent application Ser. No.14/835,679, using a known etching technique, the plurality of features216 are formed in the substrate 208, and may have a generally pyramidalshape. Notably, some of the plurality of features 216 may havecomparatively “flat” tops. The features 216 also have a height that maybe substantially the same across the width of the interface between thesubstrate 208 and the layer 209. Moreover, the width (x-dimension in thecoordinate system of FIG. 2A) of the features 216 may be the same, ormay be different. Generally, however, the width of the features is onthe order of the desired fundamental mode of the SAW resonator device200. Alternatively, and again depending on the method of fabrication,the height of the features 216 may not be the same. Rather, by selectingthe height of the features to be different, a reduction in the incidenceof more than one of the spurious modes can be realized, furtherimproving the performance of the SAW resonator device 200.

In accordance with a representative embodiment, the height of thefeatures 216 is approximately 0.2λ, to 10.0λ, where λ is the wavelengthof one or more of the spurious modes. Selecting the height of thefeatures to be approximately 0.2λ to 10.0λ, of a particular spuriousmode alters the phase of the reflected waves, and results in destructiveinterference by the reflected waves, and substantially prevents theestablishment of standing waves, and thus spurious modes.

In some embodiments, the height of the features 216 is substantially thesame, and, again, the height is selected to be in the range ofapproximately 0.2λ to 10λ, where λ is the wavelength of one (e.g., apredominant) of the spurious modes. In other embodiments, the height ofthe features 216 is not the same, but rather each feature has adifferent height that is selected to be in the range of approximately0.2λ to approximately 10λ of one of the multiple spurious modes. Byselecting one height or multiple heights, the phase of the reflectedwaves is altered, and results in destructive interference by thereflected waves, thereby substantially preventing the establishment ofstanding waves of multiple frequencies, thus preventing theestablishment of multiple spurious modes.

By way of example, if the spurious modes have a frequency of 700 MHz,the wavelength λ is approximately 6.0 μm. As such, the height of thefeatures 216 would be in the range of approximately 1.2 μm toapproximately 60 μm. By contrast, if the spurious modes have a frequencyof 4200 MHz, then λ is approximately 1.0 μm. In this example, the heightof the features 116 would be approximately 0.20 μm. More generally, theheight of the features is in the range of less than approximately 0.2 μm(e.g., 0.1 μm) to greater than approximately 1.5 μm (e.g., 10.0 μm). Aswill be appreciated, the range for the height depends on the frequencyof the fundamental mode.

The non-periodic orientation of the plurality of features 216, thegenerally angled surfaces provided by the plurality of features 116, andproviding the height of the features 216 to be in the noted rangerelative to the wavelength of the propagating spurious modes combine toalter the phase of the acoustic waves incident on the various features.Beneficially, these factors in combination result in comparativelydiffuse reflection of the acoustic waves back into the piezoelectriclayer 203. This comparatively diffuse reflection of the acoustic wavesfrom the features 216 will generally not foster constructiveinterference, and the establishment of resonance conditions.Accordingly, the plurality of features 216 generally prevent theabove-noted parasitic acoustic standing waves (i.e., spurious modes),which travel down and into the substrate 208, from being establishedfrom the acoustic waves generated in the piezoelectric layer 203, whichtravel down and into the substrate 208.

One measure of the impact of the parasitic spurious modes on theperformance of a device (e.g., filter) comprising a SAW resonator is thequality (Q) factor. For example, the parasitic spurious modes couple atthe interfaces of the piezoelectric layer 103 and remove energyavailable for the desired SAW modes and thereby reduce the Q-factor ofthe resonator device. As is known, the Q-circle of a Smith Chart has avalue of unity along its circumferences. The degree of energy loss (andtherefore reduction in Q) is depicted with the reduction of the S₁₁parameter of the unit circle. Notably, as a result of parasitic spuriousmodes and other acoustic losses, sharp reductions in Q of known devicescan be observed on a so-called Q-circle of a Smith Chart of the S₁₁parameter. These sharp reductions in Q-factor are known as “rattles,”and are strongest in the southeast quadrant of the Q-circle (i.e., forfrequencies above the anti-resonant frequency, fp). These spurious modesoccur at discrete frequencies over a range of frequencies above themechanical or fundamental frequency. While not too problematic in singleband filtering, these modes can be particularly problematic inapplications of aggregation of multiple bands to be used simultaneously(carrier aggregation). In such applications, if one filter has spuriousmodes outside its own band, they are likely to exist in the bands ofanother filter. This is unacceptable for normal carrier aggregation.Beneficially, because of the diffuse reflections of the presentteachings, and attendant phase mismatch of the reflected acoustic wavesrealized by the plurality of features 216, compared to such knowndevices, a filter comprising SAW resonator devices of representativeembodiments of the present teachings, shows lesser magnitudes of the“rattles”, and a somewhat “spreading” of the reduced “rattles” isexperienced.

FIG. 2B is a conceptual view in cross-section of a portion of the SAWresonator device 200 of FIG. 2A. Many aspects and details of SAWresonator device 200 are common to those described above in connectionwith FIGS. 1-2A. These common aspects and details may not be repeated toavoid obscuring the description of the present representativeembodiment.

Turning to FIG. 2B, the orientation of a polarization axis (C-axis) 222is disposed in the x-z plane of the coordinate system of FIG. 2B. Assuch, the C-axis 222 is illustratively parallel to the x-axis of thecoordinate system shown. Notably, and for reasons described more fullybelow, while it is desirable and beneficial for the C-axis 222 to beparallel to the x-axis, in view of fabrication constraints, this can bedifficult to attain. As such, the present teachings contemplate theC-axis 222 to be at an angle (Θ) subtended by the x and z axes in therange of approximately 0° to approximately 45° in the coordinate systemshown in FIG. 2B. As can be appreciated, the second surface 211 of thepiezoelectric layer 203 is disposed in the x-y plane, so the C-axis isat the angle (Θ) in the range of approximately 0° to approximately 45°.In this way, the component of the C-axis 222 along the z-direction (orin the x-y plane) of the coordinate system depicted in FIG. 2B is largeby comparison to the component of the C-axis oriented perpendicularthereto.

In operation, time dependent electric fields (represented by electricfield lines 224) are generated by the electrodes 202. These electric (E)fields terminate on the electrodes as shown by the electric field lines224, and have components in the x-direction in the coordinate system ofFIG. 2B, that is greatest at the local maxima 226, 228, 230. As notedabove, the C-axis of the piezoelectric layer 203 is beneficiallyparallel to the x-axis in the coordinate system of FIG. 2B, or, in viewof the maximum of the angle (Θ) being comparatively small, the magnitudeof the C-axis vector parallel to the x-axis is significantly greaterthan the C-axis vector parallel to the z-axis in the coordinate systemof FIG. 2B. Accordingly, the vector component of the E-field at allpoints parallel to the vector component of the C-axis in the x-directionof the coordinate system of FIG. 2B causes longitudinal modes to besupported in the x-direction along the x-y plane of the piezoelectriclayer 203, with alternating polarity of the E field causing alternatingexpansion and contraction of the piezoelectric layer 203. Theselongitudinal modes are bounded by the layer 209, which has a dielectricconstant that is significantly lower (e.g., ε_(r) approximately 3.9)than that of piezoelectric layer 203 (ε_(r) of approximately 11 for AlN,and higher for ASN), thereby reducing acoustic energy loss into thesubstrate 208.

As is known, in bulk acoustic wave (BAW) devices, piezoelectric layerssuch as AlN and ASN are grown to provide a highly textured materialhaving its polarization (C) axis oriented perpendicular to the x-y planeof the coordinate system of FIG. 2B. In accordance with certainrepresentative embodiments, fabricating a highly textured piezoelectriclayer such as piezoelectric layer 203 having its C-axis disposed in adirection parallel to the x-axis (or in the x-z plane) of the coordinatesystem can be done through a variation of methods used to form highlytextured piezoelectric layers in BAW devices. Notably, methods such asdescribed in commonly owned U.S. Pat. No. 9,243,316 and U.S. PatentApplication Publication No. 20140132117, the entire disclosures of whichare specifically incorporated herein by reference, may be modified toform the desired piezoelectric layer 203. One notable modification wouldbe to effect the ion-assisted sputtering at a glancing angle relative tothe target. Alternatively, the piezoelectric layer 203 with the C-axisdisposed in a direction parallel to the x-axis may be fabricatedaccording to the teachings of Kochar, et al. “NSPUDT using C-Axis tiltedScAlN Thin Film” (Frequency Control Symposium and the European Frequencyand Time Forum (FCS), 2015 Joint Conference of the IEEE International,April 2015), the entire disclosure of which is specifically incorporatedherein by reference. Alternatively, a standard magnetron can be used todeposit the AlN or ASN having a C-axis oriented orthogonally to the x-yplane of the coordinate system of FIG. 2B, however, with an ion sourcethat directs Argon atoms at a glancing angle. This inhibits the slightlypreferential C-axis crystal growth normal to the x-y plane of thecoordinate system of FIG. 2B, and encourages growth of the polarizationaxis (C-axis) in the x-z plane of the depicted coordinate system (i.e.,at an angle relative to the x,y plane).

As will be appreciated, the epitaxial layer provides a good latticestructure over which the piezoelectric layer 203 is grown. Afterdeposition of the epitaxial layer, the piezoelectric layer 203 is grownusing a known method, such as Metalorganic Vapor Phase Epitaxy (MOVPE)or Molecular Beam Epitaxy (MBE). Alternatively, a single crystalpiezoelectric wafer (e.g., single crystal AlN or single crystal ASN)with a polarization axis (C-axis) in the desired orientation may bebonded to layer 209, or to a silicon wafer (e.g., silicon layer 330, orsubstrate 408 described in connection with representative embodiments ofFIGS. 3 and 4, below).

FIG. 3 is a cross-sectional view of the SAW resonator device 300 inaccordance with a representative embodiment. Many aspects and details ofSAW resonator device 300 are common to those described above inconnection with FIGS. 1-2B. These common aspects and details may not berepeated to avoid obscuring the description of the presentrepresentative embodiment.

The SAW resonator device 300 comprises a substrate 308 disposed beneaththe piezoelectric layer 303, a layer 309 disposed over the substrate308, and a silicon layer 330 disposed between the layer 309 and thepiezoelectric layer 303.

The piezoelectric layer 303 illustratively comprises aluminum nitride(AlN) or scandium doped AlN (ASN). Generally, in the representativeembodiments described below, the piezoelectric layer 303 is a wafer thatis previously fabricated, and that is adhered to the layer 309 by atomicbonding as described more fully below.

The C-axis 322 of the piezoelectric layer 303 is illustratively parallelto the x-axis of the coordinate system shown. Alternatively, and asnoted above, the present teachings contemplate the C-axis 322 to be atan angle (Θ) subtended by the x and z axes in the range of approximately0° to approximately 45° in the coordinate system shown in FIG. 3.

In accordance with a representative embodiment, the substrate 308comprises a bulk region 308′, and a surface region 308″. As describedmore fully below, the bulk region 308′ comprises a high-resistivitymonocrystalline semiconductor material, and the surface region 308″ hasa high trap density and a reduced carrier mobility compared to the bulkregion 308′.

In accordance with a representative embodiment, the bulk region 308′comprises monocrystalline silicon, and has thickness of approximately50.0 μm to approximately 800.0 μm. In certain representativeembodiments, in order to improve the performance of a filter comprisingSAW resonator device(s) 300, the substrate 308 may comprise acomparatively high-resistivity material. Illustratively, the substrate308 may comprise single-crystal silicon that is doped to a comparativelyhigh resistivity. Notably, other high-resistivity, monocrystallinematerials besides silicon are contemplated for use as the substrate 308of the SAW resonator device 300. By way of example, other single-crystalsemiconductor materials can be used for the substrate 308. Moreover,other single-crystal materials, such as single crystal aluminum oxide(Al₂O₃) (sometimes referred to as “sapphire”) could be used as well.

The surface region 308″ comprises a material, which, compared to thebulk region 308′, has an increased bandgap, very high trap density (highprobability of trapping free charge carriers), and a reduced carriermobility.

The thickness of this (nonmonocrystalline) surface region 308″ should beat least as great as the depth of an inversion layer that would form ifthe surface region 308″ were made up of a monocrystalline material(e.g., silicon). Since the thickness of the inversion layer depends,among other things, on the semiconductor material and its doping, thethickness of the surface region 308″ is typically in range from a fewnanometers (nm) up to several hundreds of micrometers (μm).

As noted above, the bulk region 308′ may comprise single-crystalsilicon. In such an embodiment, the surface region may compriseamorphous silicon (aSi), or polycrystalline silicon. Notably, however,since a single grain of a polysilicon material is monocrystalline, thegrain size of the polycrystalline silicon here should be much smallercompared to a typical resonator device size and the thickness of thesurface region 308″. As noted above, an average grain size isillustratively more than 10 times smaller than the thickness of thesurface region.

The surface region 308″ can be formed by one of a number of knownmethods. For example, an amorphous silicon layer can be deposited by aknown method over the bulk region 308′ to form the substrate 308.Similarly, a polysilicon layer may be deposited by a known method overthe bulk region 308′ to form the substrate 308. Notably, of course, thegrain size of the polysilicon structure that comprises the surfaceregion 308″ in such an embodiment should be much smaller than thestructure sizes/area of the resonator. Moreover, known ion implantationmethods may be used to dope or amorphize the monocrystalline structureof the substrate 308, thereby creating the surface region 308″ over thebulk region 308′.

As noted above, other semiconductor materials (not silicon) can be usedas the substrate 308. By similar methods, the surface region 308″ can beprovided over the bulk region 308′ by depositing or forming adeteriorated lattice structure by similar techniques to those used forsilicon. As such, adding a layer of amorphous or polycrystallinematerial over the bulk region 308′, or deteriorating the latticestructure of a given substrate material, provides a surface region 308″comprising an increased bandgap, very high trap density and an at least100 times reduced carrier mobility, compared to the bandgap, trapdensity and carrier mobility of the bulk region 308′.

As noted above, a benefit of the certain representative embodiments isthat the use of such a substrate avoids the generation of a surfacechannel at the surface of the semiconductor, which otherwise (for aconventional substrate) is formed by an inversion layer. Such a surfacechannel results in a lossy surface current between resonators and/orinterconnections of different potential. As a consequence, SAW resonatordevices of the present teachings will provide a comparatively lowerloss, which means that the SAW filters comprise an improved insertionloss, or the SAW resonators comprise an improved quality factor (Q).

The layer 309 is illustratively an oxide material, such as SiO₂, orphosphosilicate glass (PSG), borosilicate glass (BSG), a thermally grownoxide, or other material amenable to polishing to a high degree ofsmoothness, as described more fully below. The layer 309 is deposited bya known method, such as chemical vapor deposition (CVD) or plasmaenhanced chemical vapor deposition (PECVD), or may be thermally grown.As described more fully below, the layer 309 is polished to a thicknessin the range of approximately 0.05 μm to approximately 6.0 μm.

The silicon layer 330 is illustratively polycrystalline silicon(poly-Si) and is deposited using a known method, such as plasma-enhancedchemical vapor deposition (PECVD) or a similar method. After depositionis complete, a cleaning step, such as a known sputtering step, iscarried out to remove any oxide or debris from the first surface 331 ofthe silicon layer 330. This cleaning step fosters bonding of the firstsurface 331 to the piezoelectric layer 303. This bonding provides goodadhesion between the silicon layer 330 and the piezoelectric layer 303.

The piezoelectric layer 303 may be formed using methods such as thosedescribed above in the formation of piezoelectric layer 203. By way ofexample, the piezoelectric layer 303 with the C-axis 322 disposed in adirection parallel to the x-axis of the coordinate system of FIG. 3 maybe fabricated over the silicon layer 330 by a known method that includesproviding an epitaxial layer (seed layer), having its C-axis 322 alsodisposed in a direction parallel to the x-axis, over the layer 209.After deposition of the epitaxial layer, the piezoelectric layer 303 isgrown using a known method, such as Metalorganic Vapor Phase Epitaxy(MOVPE) or Molecular Beam Epitaxy (MBE).

Illustratively, the silicon layer 330 has a thickness in range ofapproximately 100 Å to approximately one-third of the wavelength (λ/3)of a SAW wave, where the wavelength is defined by the pitch of theinterdigitated electrodes 302 (IDT) and the velocity of sound in themedium (L=v_(a)/2*pitch). Generally, the thickness of the silicon layer330 is selected to be thick enough so that it is atomically smooth andcontinuous, and not too thick that the desired scattering of spuriousmodes from the features 316 does not occur. To this end, if the siliconlayer 330 is too thin, unevenness across the thickness can result inrelative peaks and valleys across the first surface 331 and incompletecoverage. These peaks and valleys deleteriously reduce the area ofcontact between the first surface 331 of the silicon layer 330 and thesecond surface 311 of the piezoelectric layer 303. By contrast, if thesilicon layer 330 is too thick, the silicon layer 330 is like asubstrate without features 316, allowing undesired spurious modes topropagate without incoherent reflection as is realized by the structureof the present teachings.

The piezoelectric layer 303 has a first surface 310, and a secondsurface 311, which opposes the first surface 310. The layer 309 has afirst surface 312 and a second surface 313, and the silicon layer 330has a first surface 331 and a second surface 332. As depicted in FIG. 3,the first surface 331 of the silicon layer 330 is atomically bonded tothe second surface 311 of the piezoelectric layer 303, as described morefully below.

The substrate 308 has a first surface 314 and a second surface 315opposing the first surface 314. The first surface 314 has a plurality offeatures 316 there-across. As noted above, undesired spurious modes arelaunched in the piezoelectric layer 303, and propagate down to the firstsurface 314. As described more fully in above-incorporated U.S. patentapplication Ser. No. 15/009,801, the plurality of features 316 reflectundesired spurious modes at various angles and over various distances todestructively interfere with the undesired spurious waves in thepiezoelectric layer 303, and possibly enable a portion of these waves tobe beneficially converted into desired SAW waves. Again as describedmore fully below, the reflections provided by the plurality of features316 foster a reduction in the degree of spurious modes (i.e., standingwaves), which are created by the reflection of acoustic waves at theinterface of the second surface 311 of the piezoelectric layer 303 andthe first surface 312 of layer 309. Ultimately, the reflections providedby the plurality of features 316 serve to improve the performance ofdevices (e.g., filters) that comprise a plurality of SAW resonatordevices 300.

As described in above-incorporated U.S. patent application Ser. No.15/009,801, there are multiple spurious modes, each having a differentfrequency and wavelength. In accordance with a representativeembodiment, the height of the features 316 of the substrate 308 isapproximately 0.2λ to 10.0λ, where λ is the wavelength of one or more ofthe spurious modes. Selecting the height of the features to beapproximately 0.2λ to 10.0λ of a particular spurious mode alters thephase of the reflected waves, and results in destructive interference bythe reflected waves, and substantially prevents the establishment ofstanding waves, and thus spurious modes.

In some embodiments, the height of the features 316 is substantially thesame, and the height is selected to be in the range of approximately0.2λ to 10λ, where λ is the wavelength of one (e.g., a predominant) ofthe spurious modes. In other embodiments, the height of the features 316is not the same, but rather each feature has a different height that isselected to be in the range of approximately 0.2λ to approximately 10λof one of the multiple spurious modes. By selecting one height ormultiple heights, the phase of the reflected waves is altered, andresults in destructive interference by the reflected waves, therebysubstantially preventing the establishment of standing waves of multiplefrequencies, thus preventing the establishment of multiple spuriousmodes.

As described more fully in above-incorporated U.S. patent applicationSer. No. 15/009,801, in accordance with a representative embodiment, thefirst (upper) surface 331 of silicon layer 330 is polished, such as bychemical-mechanical polishing in order to obtain a “mirror” like finishwith a comparatively low root-mean-square (RMS) variation of height.This low RMS variation of height significantly improves the contact areabetween the first surface 331 of the silicon layer 330 and the secondsurface 311 of the piezoelectric layer 303. Accordingly, providing a lowRMS variation in height improves the atomic bonding between the firstsurface 331 and the second surface 311. As is known, the bond strengthrealized by atomic bonding is directly proportional to the contact areabetween two surfaces. As such, improving the flatness/smoothness of thefirst surface 331 fosters an increase in the contact area, therebyimproving the bond of the silicon layer 330 to the piezoelectric layer303. As used herein, the term “atomically smooth” means sufficientlysmooth to provide sufficient contact area to provide a sufficientlystrong bond strength between the silicon layer 330 and the piezoelectriclayer 303, at the interface of their first and second surfaces 331, 311,respectively.

In operation, and as described above, time dependent electric fields(not shown in FIG. 3) are generated by the interdigitated electrodes302. These electric (E) fields terminate on the electrodes, and havecomponents in the x-direction in the coordinate system of FIG. 3, thatis greatest at the local maxima of the E field. As noted above, theC-axis 322 of the piezoelectric layer 303 is beneficially parallel tothe x-axis in the coordinate system of FIG. 3, or, in view of themaximum of the angle (Θ) being comparatively small, the magnitude of theC-axis vector parallel to the x-axis is significantly greater than theC-axis vector parallel to the z-axis in the coordinate system of FIG. 3.Accordingly, the vector component of the E-field at all points parallelto the vector component of the C-axis 322 in the x-direction of thecoordinate system of FIG. 3 causes longitudinal modes to be supported inthe x-direction along the x-y plane of the piezoelectric layer 303, withalternating polarity of the E field causing alternating expansion andcontraction of the piezoelectric layer 303. These longitudinal modes arebounded by the layer 309, which has a dielectric constant that issignificantly lower (e.g., ε_(r) of approximately 3.9) than that ofpiezoelectric layer 203 (e.g., ε_(r) of approximately 11 for AlN, andhigher for ASN), thereby reducing acoustic energy loss into thesubstrate 308.

It is noted that the polishing sequence described above to provide thedesired smoothness of the first surface 331 of the silicon layer 330 maybe foregone if the deposition sequence used to form the silicon layer330 results in an atomically smooth first surface 331.

FIG. 4 is a cross-sectional view of the SAW resonator device 400. Manyaspects and details of SAW resonator device 400 are common to thosedescribed above in connection with FIGS. 1-3. These common aspects anddetails may not be repeated to avoid obscuring the description of thepresent representative embodiment.

The SAW resonator device 400 comprises a substrate 408 disposed beneaththe piezoelectric layer 403. The piezoelectric layer 403 illustrativelycomprises aluminum nitride (AlN) or scandium doped AlN (ASN). Generally,in the representative embodiments described below, the piezoelectriclayer 403 is adhered to the substrate 408 by atomic bonding.

The C-axis 422 of the piezoelectric layer 403 is illustratively parallelto the x-axis of the coordinate system shown. Alternatively, and asnoted above, the present teachings contemplate the C-axis 422 to be atan angle (Θ) subtended by the x and z axes in the range of approximately0° to approximately 45° in the coordinate system shown in FIG. 4.

In accordance with a representative embodiment, the substrate 408comprises a bulk region 408′, and a surface region 408″. The bulk regioncomprises a high-resistivity monocrystalline semiconductor material, andthe surface region 408″ has a high trap density and a reduced carriermobility compared to the bulk region 408′.

In accordance with a representative embodiment, the bulk region 408′comprises monocrystalline silicon, and has a thickness of approximately50.0 μm to approximately 800.0 μm. In certain representativeembodiments, in order to improve the performance of a filter comprisingSAW resonator device(s) 400, the substrate 408 may comprise acomparatively high-resistivity material. Illustratively, the substrate408 may comprise single-crystal silicon that is doped to a comparativelyhigh resistivity. Notably, other high-resistivity, monocrystallinematerials besides silicon are contemplated for use as the substrate 408of the SAW resonator device 400. By way of example, other single-crystalsemiconductor materials can be used for the substrate 408. Moreover,other single-crystal materials, such as single-crystal aluminum oxide(Al₂O₃) (sometimes referred to as “sapphire”) could be used as well.

The surface region 408″ comprises a material, which, compared to thebulk region 408′, has an increased bandgap, very high trap density (highprobability of trapping free charge carriers), and a reduced carriermobility.

The thickness of this (nonmonocrystalline) surface region 408″ should beat least as great as the depth of an inversion layer that would form ifthe surface region 408″ were made up of a monocrystalline material(e.g., silicon). Since the thickness of the inversion layer depends,among other things, on the semiconductor material and its doping, thethickness of the surface region 408″ is typically in range from a fewnanometers (nm) up to several hundreds of micrometers (μm).

As noted above, the bulk region 408′ may comprise single-crystalsilicon. In such an embodiment, the surface region may compriseamorphous silicon (aSi), or polycrystalline silicon. Notably, however,since a single grain of a polysilicon material is monocrystalline, thegrain size of the polycrystalline silicon here should be much smallercompared to a typical resonator device size and the thickness of thesurface region 408″.

The surface region 408″ can be formed by one of a number of knownmethods. For example, an amorphous silicon layer can be deposited by aknown method over the bulk region 408′ to form the substrate 408.Similarly, a polysilicon layer may be deposited by a known method overthe bulk region 408′ to form the substrate 408. Notably, of course, thegrain size of the polysilicon structure that comprises the surfaceregion 408″ in such an embodiment should be much smaller than thestructure sizes/area of the resonator. Moreover, known ion implantationmethods may be used to dope, or amorphize or otherwise deteriorate themonocrystalline lattice structure of the substrate 408, thereby creatingthe surface region 408″ over the bulk region 408′.

As noted above, other semiconductor materials (not silicon) can be usedas the substrate 408. By similar methods, the surface region 408″ can beprovided over the bulk region 408′ by depositing or forming adeteriorated lattice structure by similar techniques to those used forsilicon. As such, adding a layer of amorphous or polycrystallinematerial over the bulk region 408′, or deteriorating the latticestructure of a given substrate material, provides a surface region 408″comprising an increased bandgap, very high trap density and an at least100 times reduced carrier mobility, compared to the bandgap, trapdensity and carrier mobility of the bulk region 408′.

As noted above, a benefit of the certain representative embodiments isthat the use of such a substrate avoids the generation of a surfacechannel at the surface of the semiconductor, which otherwise (for aconventional substrate) is formed by an inversion layer. Such a surfacechannel results in a lossy surface current between resonators and/orinterconnections of different potential. In the presently describedrepresentative embodiment in which no oxide or other planarization, oratomic bonding layer is provided, an inversion layer near the surface ofthe substrate 408 would be created. The surface region 408″ serves tosuppress the creation of this inversion layer. As a consequence, SAWresonator device 400 of the present teachings will provide acomparatively lower loss, which means that the SAW filters comprise animproved insertion loss, or the SAW resonators comprise an improvedquality factor (Q).

In operation, and as described above, time dependent electric fields(not shown in FIG. 3) are generated by the electrodes 402. Theseelectric (E) fields terminate on the electrodes 402, and have componentsin the x-direction in the coordinate system of FIG. 4, that is greatestat the local maxima of the E field. As noted above, the C-axis 422 ofthe piezoelectric layer 403 is beneficially parallel to the x-axis inthe coordinate system of FIG. 4, or, in view of the maximum of the angle(Θ) being comparatively small, the magnitude of the C-axis vectorparallel to the x-axis is significantly greater than the C-axis vectorparallel to the z-axis in the coordinate system of FIG. 4. Accordingly,the vector component of the E-field at all points parallel to the vectorcomponent of the C-axis 422 in the x-direction of the coordinate systemof FIG. 4 causes longitudinal modes to be supported in the x-directionalong the x-y plane of the piezoelectric layer 403, with alternatingpolarity of the E field causing alternating expansion and contraction ofthe piezoelectric layer 403.

When connected in a selected topology, a plurality of SAW resonators canfunction as an electrical filter. FIG. 5 shows a simplified schematicblock diagram of an electrical filter 500 in accordance with arepresentative embodiment. The electrical filter 500 comprises seriesSAW resonators 501 and shunt SAW resonators 502. The series SAWresonators 501 and shunt SAW resonators 502 may each comprise SAWresonator devices 100 (or SAW resonator devices 200, or 300, or 400)described in connection with the representative embodiments of FIGS.1-4. As can be appreciated, the SAW resonator devices (e.g., a pluralityof SAW resonator devices 100) that comprise the electrical filter 500may be provided over a common substrate (e.g., substrate 208), or may bea number of individual SAW resonator devices (e.g., SAW resonatordevices 100) disposed over more than one substrate (e.g., more than onesubstrate 208). The electrical filter 500 is commonly referred to as aladder filter, and may be used for example in duplexer applications. Itis emphasized that the topology of the electrical filter 500 is merelyillustrative and other topologies are contemplated. Moreover, theacoustic resonators of the representative embodiments are contemplatedin a variety of applications including, but not limited to duplexers.

The various components, materials, structures and parameters areincluded by way of illustration and example only and not in any limitingsense. In view of this disclosure, those skilled in the art canimplement the present teachings in determining their own applicationsand needed components, materials, structures and equipment to implementthese applications, while remaining within the scope of the appendedclaims.

The invention claimed is:
 1. A surface acoustic wave (SAW) resonatordevice, comprising: a semiconductor substrate having a first surface anda second surface; a piezoelectric layer disposed over the semiconductorsubstrate, the piezoelectric layer having a first surface and a secondsurface, wherein a polarization axis (C-axis) of the piezoelectric layeris oriented at an angle relative to the second surface of thepiezoelectric layer in a range of approximately 0° to approximately 45°;a plurality of electrodes disposed over the first surface of thepiezoelectric layer, the plurality of electrodes configured to generatelongitudinal acoustic waves in the piezoelectric layer; and a layerhaving a first surface and a second surface, the layer being disposedbetween the first surface of the semiconductor substrate and the secondsurface of the piezoelectric layer, wherein the first surface of thelayer has a smoothness sufficient to foster atomic bonding between thefirst surface of the layer and the second surface of the piezoelectriclayer.
 2. The SAW resonator device as claimed in claim 1, wherein thepiezoelectric layer is doped with a rare-earth element.
 3. The SAWresonator device as claimed in claim 1, wherein the piezoelectric layercomprises aluminum nitride (AlN).
 4. The SAW resonator device as claimedin claim 3, wherein the piezoelectric layer is doped with scandium (Sc).5. The SAW resonator device as claimed in claim 1, wherein the layercomprises an oxide material.
 6. The SAW resonator device as claimed inclaim 5, wherein the oxide material comprises silicon dioxide (SiO₂). 7.The SAW resonator device as claimed in claim 1, the second surface ofthe layer having a smoothness sufficient to foster atomic bondingbetween the second surface of the layer and the first surface of thesemiconductor substrate.
 8. The SAW resonator device as claimed in claim1, wherein the first surface of the layer has a root-mean-square (RMS)variation in height of approximately 0.1 Å to approximately 10.0 Å. 9.The SAW resonator device as claimed in claim 1, wherein thesemiconductor substrate comprises a bulk region and a surface region,wherein the surface region has a high trap density and a reduced carriermobility compared to the bulk region.
 10. The SAW resonator as claimedin claim 9, wherein the bulk region is a substantially monocrystallinesemiconductor, and the surface region is a substantially amorphoussemiconductor, or substantially polycrystalline semiconductor.
 11. TheSAW resonator as claimed in claim 9, wherein the surface region has athickness that exceeds a thickness of an inversion layer in a knownsemiconductor substrate, which has the layer disposed thereon.
 12. TheSAW resonator device of claim 9, wherein the surface region has athickness within a range of approximately 1 nm and approximately 700 um.13. The SAW resonator device of claim 9, wherein the surface regioncomprises a layer of amorphous semiconductor material disposed over thebulk region of the semiconductor substrate.
 14. The SAW resonator deviceof claim 9, wherein the surface region comprises implanted ion atoms,which deteriorate a monocrystalline lattice structure of thesemiconductor substrate.
 15. The SAW resonator device as claimed inclaim 1, wherein the first surface of the semiconductor substratecomprises a plurality of features, and the plurality of features reflectacoustic waves and reduce an incidence of spurious modes in thepiezoelectric layer.
 16. A surface acoustic wave (SAW) resonator device,comprising: a semiconductor substrate having a first surface and asecond surface; a piezoelectric layer disposed over the semiconductorsubstrate, the piezoelectric layer having a first surface and a secondsurface, wherein a polarization axis (C-axis) of the piezoelectric layeris oriented at an angle relative to the second surface of thepiezoelectric layer in a range of approximately 0° to approximately 45°;a plurality of electrodes disposed over the first surface of thepiezoelectric layer, the plurality of electrodes configured to generatelongitudinal acoustic waves in the piezoelectric layer; a layer having afirst surface and a second surface, the layer being disposed between thefirst surface of the semiconductor substrate and the second surface ofthe piezoelectric layer; and a silicon layer disposed between the firstsurface of the layer and the second surface of the piezoelectric layer,a first surface of the silicon layer having a smoothness sufficient tofoster atomic bonding between the first surface of the silicon layer andthe second surface of the piezoelectric layer, wherein the first surfaceof the semiconductor substrate comprises a plurality of features, andthe plurality of features reflect acoustic waves and reduce an incidenceof spurious modes in the piezoelectric layer.
 17. The SAW resonatordevice as claimed in claim 16, wherein the features are substantiallynot in a regular pattern.
 18. The SAW resonator device as claimed inclaim 16, the semiconductor substrate comprising a bulk region and asurface region wherein: the surface region has a high trap density and areduced carrier mobility compared to the bulk region, wherein thesemiconductor substrate has a first surface and a second surface; andthe surface region comprises a same material as the semiconductorsubstrate.
 19. A SAW resonator as claimed in claim 18, wherein the bulkregion is a substantially monocrystalline semiconductor, and the surfaceregion is substantially amorphous semiconductor, or substantiallypolycrystalline semiconductor.
 20. A SAW resonator as claimed in claim18, wherein the surface region has a thickness that exceeds a thicknessof an inversion layer in a known semiconductor substrate, which has thelayer disposed thereon.
 21. The SAW resonator device of claim 18,wherein the surface region has a thickness within a range ofapproximately 1 nm and approximately 700 um.
 22. The SAW resonatordevice of claim 18, wherein the surface region comprises polycrystallinematerial with an average grain size of more than 10 times smaller than athickness of the surface region.
 23. The SAW resonator device of claim22, wherein an average grain size is at least 10 times smaller than anarea of a resonator.
 24. The SAW resonator device of claim 18, whereinthe surface region comprises a layer of amorphous semiconductor materialdisposed over the bulk region of the semiconductor substrate.
 25. TheSAW resonator device of claim 18, wherein the surface region comprisesimplanted ion atoms, which deteriorate a monocrystalline latticestructure of the semiconductor substrate.